The present invention generally relates to ink delivery systems, and more particularly to a thermal inkjet printhead which is characterized by improved reliability, increased longevity, diminished production costs, greater versatility, cooler printhead operating temperatures, decreased energy consumption (e.g. diminished "turn-on-energy" ["TOE"] requirements), and greater overall printing efficiency. The claimed invention likewise allows the production of printhead systems having many different size characteristics including the fabrication of "wide-array" devices. These goals are accomplished through the use of a novel polycrystalline silicon resistor system which uses a minimal number of material layers that are specially treated during production to enable various portions of the layers to perform different functions. In this manner, a simple yet highly "differentiated" structure is produced. Likewise, the completed resistor elements are located in a "buried" configuration within the printhead as discussed in considerable detail below.
Substantial developments have been made in the field of electronic printing technology. A wide variety of highly-efficient printing systems currently exist which are capable of dispensing ink in a rapid and accurate manner. Thermal inkjet systems are especially important in this regard. Printing units using thermal inkjet technology basically involve an apparatus which includes at least one ink reservoir chamber in fluid communication with a substrate (preferably made of silicon [Si] and/or other comparable materials) having a plurality of thin-film heating resistors thereon. The substrate and resistors are maintained within a structure that is conventionally characterized as a "printhead". Selective activation of the resistors causes thermal excitation of the ink materials stored inside the reservoir chamber and expulsion thereof from the printhead. Representative thermal inkjet systems are discussed in U.S. Pat. No. 4,500,895 to Buck et al.; U.S. Pat. No. 4,771,295 to Baker et al.; U.S. Pat. No. 5,278,584 to Keefe et al.; and the Hewlett-Packard Journal, Vol. 39, No. 4 (August 1988), all of which are incorporated herein by reference.
The ink delivery systems described above (and comparable printing units using thermal inkjet technology) typically include an ink containment unit (e.g. a housing, vessel, or tank) having a self-contained supply of ink therein in order to form an ink cartridge. In a standard ink cartridge, the ink containment unit is directly attached to the remaining components of the cartridge to produce an integral and unitary structure wherein the ink supply is considered to be "on-board" as shown in, for example, U.S. Pat. No. 4,771,295 to Baker et al. However, in other cases, the ink containment unit will be provided at a remote location within the printer, with the ink containment unit being operatively connected to and in fluid communication with the printhead using one or more ink transfer conduits. These particular systems are conventionally known as "off-axis" printing units. Representative, non-limiting off-axis ink delivery systems are discussed in co-owned pending U.S. patent application No. 08/869,446 (filed on Jun. 5, 1997) entitled "AN INK CONTAINMENT SYSTEM INCLUDING A PLURAL-WALLED BAG FORMED OF INNER AND OUTER FILM LAYERS" (Olsen et al.) and U.S. Pat. No. 5,975,686 to Hauck et al. which are each incorporated herein by reference. The present invention is applicable to both on-board and off-axis systems (as well as any other types which include at least one ink containment vessel that is either directly or remotely in fluid communication with a printhead containing at least one ink-ejecting resistor therein as will become readily apparent from the discussion provided below.)
Regardless of the particular ink delivery system being employed, an important factor to consider involves the overall operating efficiency of the printhead with particular reference to the resistor elements that are used to expel ink on-demand during printhead operation. The term "operating efficiency" shall collectively encompass a number of different items including but not limited to internal temperature levels, thermal uniformity, ink delivery speed, expulsion frequency, energy requirements (e.g. current consumption), and the like. Typical and conventional resistor elements used for ink ejection in a thermal inkjet printhead are produced from a number of compositions including but not limited to a mixture of elemental tantalum [Ta] and elemental aluminum [Al] (also known as "TaAl"), as well as other comparable materials including tantalum nitride ("Ta.sub.2 N"). Polycrystalline silicon may likewise be employed in thermal inkjet printing devices, with the term "polycrystalline silicon" being generally used in a conventional manner to describe a silicon material which basically contains an aggregate of multiple individual crystals. Standard ink delivery resistor systems are discussed in considerable detail in U.S. Pat. No. 4,535,343 to Wright et al. and U.S. Pat. No. 5,122,812 to Hess et al. which are each incorporated herein by reference. Also of importance is a characteristic known as "production efficiency" which is generally defined herein to encompass the number of manufacturing steps, procedures, material layers, and the like which are needed to produce the desired resistor/printhead assembly.
However, the chemical and physical characteristics of the resistor elements and interconnection components associated therewith which are selected for use in a thermal inkjet printhead will directly influence the overall operating efficiency of the printhead (and can likewise affect the general degree of production efficiency). The terms "interconnection components", "interconnect components", "interconnection structures" and the like as employed herein generally involve the conductive traces and related elements which electrically connect the resistors to the printing control circuitry of the system (e.g. on-board or printer-based drive transistors and the like). The specific printing control circuitry which is chosen for use will depend on the type of printing apparatus under consideration. As discussed further below, the claimed invention shall not be restricted to any particular control systems and instead involves a novel arrangement of resistors and interconnect components designed to provide substantially improved operating/production efficiency.
In any thermal inkjet printing system, it is especially important that the resistor elements (and interconnect components associated therewith) be as energy efficient as possible and capable of operating at low current levels. Resistive compounds having high current requirements are typically characterized by numerous disadvantages including a need for high cost, high-current power supplies in the printer unit under consideration. Likewise, additional losses of electrical efficiency can occur which result from the passage of greater current levels through the electrical interconnect components/structures discussed above that are attached to the resistor(s), with such interconnect structures exhibiting "parasitic resistances". These parasitic resistances cause increased energy losses as greater current levels pass through the above-listed components, with such energy losses being reduced when current levels are diminished. Likewise, high current requirements in the resistor elements and the "parasitic resistances" mentioned above can result in (1) greater overall temperatures within the printhead (with particular reference to the substrate or "die" on which the printhead components are positioned [discussed further below]); and (2) lower printhead reliability/longevity levels.
Another important consideration in the development of an efficient thermal inkjet printhead is the avoidance of a condition conventionally known as "current crowding". This term shall be generally defined to involve a situation where current flow within a conductor or across an interface between two materials becomes highly non-uniform. As a result, current flow occurs within a small area of the conductor causing a very high current density (in amperes per unit area). Thus, in summary, "current crowding" is caused by a variation in some material property which results in the flow of current preferentially across a small area. For example, consider a situation where electrical current flows along a cylindrically-shaped conductor which will result in the heating of the conductor. If the conductor resistance decreases with increasing temperature, then the outside surface of the conductor will have a higher resistance compared with the center of the structure (because the outside surface can exchange heat more readily with the environment). In this manner, "current crowding" will occur since most of the current will try to flow near the low-resistance center of the structure.
"Current crowding" can reduce the overall reliability in a thermal inkjet resistor system. Depending on the degree to which "current crowding" occurs, a number of different problems can result. Severe current crowding can cause very high local current density and excessive heat generation. If the resulting heat is not effectively removed, the material being heated can melt. "Current crowding" can also lead to problems involving "electromigration". Specifically, in such a situation, "current crowding" causes high current density (but not enough to melt the structure under consideration) which nonetheless results in the physical movement of ions within the structure in the direction of electron flow. This situation can cause the structure of interest to deteriorate and otherwise experience a decrease in functionality. Further information regarding "current crowding" and the particular manner in which the present invention controls/minimizes this problem will be discussed further below in the Detailed Description of Preferred Embodiments section.
Another problem in printhead systems which contain metal structures (e.g. traces) that directly abut the resistor(s) is heat loss. Typically, metals such as aluminum have a very high thermal conductivity. As a result, a portion of the heat generated by the resistor(s) will be lost via heat conduction through abutting metal structures. To compensate for this heat loss, additional energy must be employed in the resistor assembly, thereby reducing operational efficiency.
A further important consideration in the development and production of highly-effective resistor-based thermal inkjet printheads is the ability to precisely control the thermal characteristics of the resistors in question with particular reference to (1) the resistances which can be generated per unit volume of resistor material; (2) the variation in resistance which can occur during production (with as little variation as possible being desirable); and (3) the employment of materials having favorable "TCR" levels, with the term "TCR" being conventionally defined herein to involve the "temperature coefficient of resistance". Desirable "TCR" levels in the thermal inkjet printhead/resistor systems discussed herein are optimally positive and as close to zero as possible (which is achievable using the present invention as outlined below). The "temperature coefficient of resistance" of a substance is generally defined to involve the change in resistivity of a substance per unit change in temperature. An additional item to be included in the above-described list of beneficial characteristics is the manufacture of resistor elements with low "TOE" ("turn-on-energy") requirements as mentioned above. The "TOE" associated with a given resistor in a thermal inkjet printhead is typically defined to involve the amount of energy that is required to cause the resistor of interest to generate sufficient heat for ink expulsion (which typically occurs at a representative and non-limiting temperature of about 300.degree. C. or higher). Low "TOE" levels (which are beneficial) facilitate reduced energy consumption and more rapid operation (with particular reference to "cycling" between ink expulsion "pulses").
Finally, in designing a thermal inkjet printhead, the overall topography (namely, the structural surface geometry) of the resistor(s) and associated interconnect structures must be carefully considered. Printhead designs which incorporate a substantial number of non-planar, angular, and/or sloped components in direct proximity with the resistor elements can cause various problems. Specifically, printhead units which employ structures of this nature may be more difficult to effectively cover with the passivation coverings and other layers that are normally used to protect the resistor(s) and adjacent components from corrosion. As a result, these protective layers are more prone to various defects including but not limited to cracks, "pinholes", and the like. It is therefore desirable to employ a printhead design which avoids the use of material layers having sloped sidewalls and other geometrical complexities in the vicinity of the resistor elements.
In accordance with the information provided above, it is readily apparent that a number of important factors must be considered in the development of a thermal inkjet printhead having a maximum degree of operational efficiency. While prior printhead designs of the type discussed herein have functioned adequately, the foregoing disadvantages leave room for improvement. In this regard, a need remained (prior to development of the present invention) for a resistor system suitable for use in thermal inkjet printing systems which is capable of high efficiency/low current operation that avoids or otherwise minimizes the problems discussed above and likewise enables the beneficial characteristics described herein to be achieved. The present invention satisfies this need by providing a novel resistor system which represents a substantial improvement over previous designs. The claimed resistor system, its architecture, and the material layers/fabrication stages associated therewith offer numerous advantages including the following items: (1) improved overall reliability, stability, and longevity levels in connection with the printhead and resistor elements based on the improvements recited below; (2) the avoidance of heating efficiency problems which can lead to resistor "hot spots", absolute limits on resistance, and the like; (3) the ability to place more resistors within a given printhead in view of the reduced operating temperatures and other factors listed herein which facilitates the reduced-cost production of large-area printheads; (4) the ability to fabricate resistor structures having resistance values that are substantially independent of material thickness in connection with the deposited material layers (which is accomplished using the "buried resistor approach" outlined herein); (5) favorable "TCR"/"TOE" values; (6) the control/minimization of "current crowding" and other related problems as defined above, with this benefit leading to improved electrical efficiency; (7) reductions in printhead operating temperatures; (8) the general promotion of more favorable temperature conditions within the printhead (which result from reduced current requirements that correspondingly decrease current-based parasitic heat losses from the interconnect structures operatively attached to the resistors); (9) the ability to employ a simplified, substantially planar internal printhead design (with particular reference to the resistor element[s] and associated interconnection components) which enables more effective coverage of these items by one or more protective layers; and (10) generally superior long-term operating performance.
In accordance with the detailed information provided below, the present invention involves a thermal inkjet printhead having a novel resistor system which is unique in structure, arrangement, and functional capability. Also encompassed within the invention is an ink delivery system using the claimed printhead and specialized manufacturing methods for producing the printhead. Each of these developments will be outlined in considerable detail herein. Accordingly, the present invention represents a significant advance in thermal inkjet technology which ensures high levels of operating efficiency, excellent image quality, rapid throughput, and increased longevity/reliability.